cosmetic: doc: driver-model: Do not number driver lists

Everytime a dead driver is removed from the list,
we must re-number. This is a painful task.

Try
  git show e5323225 -- doc/driver-model/UDM-serial.txt
  git show 6f62f420 -- doc/driver-model/UDM-serial.txt
  git show b9f4bc34 -- doc/driver-model/UDM-serial.txt
to see what I mean.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>